2. MULTI-COMPONENT X-RAY EMISSION

Cygnus A is sufficiently powerful and close to show
a full array of X-ray emission components. Its surrounding cluster
emission is strong, but when modelled and subtracted from the ~ 0.2 - 2
keV ROSAT High Resolution Imager (HRI) image,
Carilli et
al. (1994)
found pronounced soft-excess emission associated with the radio hotspots,
core, and two (possibly three) regions around the limb of the lobe
plasma, together with an X-ray deficit in the inner lobes. With
reference to a standard model for a powerful radio source with
supersonic jet (Fig. 1), it is interesting to
speculate that the excesses around the lobes are parts of ring structures
pinching the contact discontinuity. There was no evidence for
increased X-ray emission due to higher gas density ahead of the beam,
in the cocoon between the contact discontinuity (containing the radio
lobes) and bow shock, as might confirm the standard model, but
Carilli et
al. (1994)
argue that the increased luminosity due to
higher density may be offset by the effect of heating, which would
tend to remove X-ray emission to an energy band above that to which
ROSAT is sensitive.

Figure 1. Sketch of the termination region
of a powerful radio jet
viewed in the rest frame of the bow shock. Radio lobe emission
fills the region inside the contact discontinuity. Between the
contact discontinuity and the bow shock we expect the ambient
X-ray-emitting medium to be both compressed and heated with respect
to the medium in front of the bow shock.

X-ray spectroscopy in the 2-10 keV energy band finds a poor fit to
cluster gas alone, and argues for the presence of non-thermal emission
seen through a large absorbing column, NH ~ 4 ×
1023 cm-2, and interpreted as emission from a heavily
obscured central AGN
(Arnaud et
al. 1987,
Ueno et al. 1994).
Interestingly, this absorbed core emission cannot be the soft-X-ray core
excess in the ROSAT HRI image
(Harris et
al. 1994b),
because such a high column density
has a disastrous effect on soft X-rays (Fig. 2).
Instead, the soft X-rays may arise from a central region in the radio source
where the only line-of-sight absorption is the Galactic column,
NH ~ 3 × 1021
cm-2. Indirect support for
this suggestion comes from the fact that the ratio of the unabsorbed
X-ray to core-radio luminosity is then very similar to that of
core-dominated quasars and those high-redshift counterparts of
Cygnus A for which the core soft X-ray emission is
separated from cluster emission (Fig. 3),
although the cluster-scale cooling flow in Cygnus A
(Reynolds &
Fabian 1996)
should contribute at some level to the HRI soft X-ray core excess.

Figure 2. As the excess (intrinsic) column
density rises to more than a few 1022 cm-2, the
counts measured with ROSAT quickly
fall. The example is for the PSPC detector, assuming a source with
a power-law spectrum of
= 1.0 (f-) and galactic column
density of 1.1 × 1020
cm-2, but a similar situation applies to the HRI and its
measurement of the X-ray core of Cygnus A.

Figure 3. The two high-redshift (z
> 0.6) radio galaxies for which
core soft X-ray emission is separated from cluster emission,
3C 280
(Worrall et
al. 1994)
and 3C 220.1
(Hardcastle et
al. 1998b),
are roughly consistent with an extrapolation of the core radio/X-ray
correlation for core-dominated quasars of comparable redshift
(Worrall et
al. 1994).
Since these quasars are believed through Unification
models to be powerful radio galaxies oriented with their jets in the
line of sight (e.g.
Barthel 1989),
the correlation supports the
interpretation of the core soft X-ray emission from these radio
galaxies as being beamed and associated with the radio jet. Cygnus A,
although local, has comparable radio-core luminosity to 3C 280, and
fits remarkably well on the correlation when the HRI core X-ray
emission is interpreted as radio-related.

Because radio galaxies are multi-component X-ray emitters, the
energy-band, sensitivity, and spatial and spectral resolution of the
observing instrument influence what is measured. Focussing X-ray
optics have the major advantage of decreasing the background, and so
Einstein was the first mission to detect some tens of radio
galaxies in soft X-rays (e.g.
Fabbiano et al. 1984)
and to separate components in nearby objects such as Cen A and M 87
(Feigelson et
al. 1981,
Schreier et
al. 1982).
The mission contributing most to the subject
over the last decade has been ROSAT. The combination of improved
sensitivity (roughly twice Einstein's collecting area at ~
1 keV) and a longer mission have permitted relatively large samples
of radio galaxies to be studied, and the point response function (PRF)
of ~ 15" Half Energy Width (HEW) for the Position Sensitive
Proportional Counter (PSPC) and ~ 4" HEW for the HRI (although
with poorer sensitivity and no spectral resolution) has led to
component separation in many sources for which pointed observations
were made. An object can look rather different when viewed with the
PSPC and HRI, with the former emphasizing extended emission and the
latter the compact components (Fig 4). ASCA has
increased the number of radio galaxies with spectroscopic
component separation (e.g.
Sambruna et
al. 1999)
although, with its relatively poor spatial resolution (HEW
3'), weak sources
often give ambiguous results, with different combinations of spectral
models giving similarly good fits to the data. The payload of
BeppoSax covers the broad energy range from soft X-rays to 300 keV,
although mostly with non-focussing optics
(Boella et
al. 1997);
its strengths are therefore in broad-band studies of bright beamed
counterparts of radio galaxies, although there are tentative claims
for the detection of heavily-obscured AGN nuclei (as for Cygnus A) in some FRIs
(Trussoni et
al. 1998).

Figure 4. The ROSAT PSPC and HRI are
sensitive to different structures in a complex source, such as J2310-437
(Worrall et
al. 1999),
a BL Lac
object in an X-ray cluster. The PSPC is more sensitive to low surface
brightness extended structures (left panel shows X-ray contours from a
4 ks PSPC exposure), whilst the superior spatial resolution of the HRI
pin-points features which are unresolved or of small spatial scale
(right panel shows 31 ks HRI exposure). Lowest contours are at
3.8, and grey-scale is
R-band CCD image.

Observational biases occur from redshift effects, not only in the
sense that in flux-limited samples the more distant sources are the
more powerful. Extended X-ray emission tends to be seen around
high-redshift radio sources only if it is of cluster size and
strength; around low-redshift sources it is easier to detect the more
compact gaseous components than larger-scale emission which fills the
detector field of view. Spectral measurements attempt to
measure excess absorption over that in the line of sight in our
Galaxy, and small fitted excesses become large intrinsic excesses when
transformed to the rest frame of a high-redshift source, where this
would not happen for a more local source.